3O5+δ cathode on Ce1.9Gd0.1O1.95 electrolyte for IT-SOFCs

Electrochemistry Communications 11 (2009) 2085–2088

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Enhancement of electrochemical performance and thermal compatibility of GdBaCo2/3Fe2/3Cu2/3O5+d cathode on Ce1.9Gd0.1O1.95 electrolyte for IT-SOFCs Seung Hwan Jo 1, P. Muralidharan 1, Do Kyung Kim * Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

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Article history: Received 26 May 2009 Received in revised form 16 July 2009 Accepted 31 August 2009 Available online 3 September 2009 Keywords: Cathode Citrate combustion synthesis Double-perovskite Electrochemical performance IT-SOFCs

a b s t r a c t Transition-metal doped double-perovskite structure oxides GdBaCo2/3Fe2/3Ni2/3O5+d (FN-GBCO), GdBaCo2/3 Fe2/3Cu2/3O5+d (FC-GBCO), GdBaCoCuO5+d (C-GBCO) and pristine GdBaCo2O5+d (GBCO) were synthesized via a citrate combustion method. The thermal-expansion coefficient (TEC) and electrochemical performance of the oxides were investigated as potential cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The TEC exhibited by the FC-GBCO cathode up to 900 °C is 14.6  10 6 °C 1, which is lower than the value of GBCO (19.9  10 6 °C 1). Area specific resistances (ASR) of 0.165 X cm2 at 700 °C and 0.048 X cm2 at 750 °C were achieved for the FC-GBCO cathode on a Ce0.9Gd0.1O1.95 (CGO) electrolyte. An electrolyte supported (300 lm thick) single-cell configuration of FC-GBCO/CGO/Ni-CGO attained a maximum power density of 435 mW cm 2 at 700 °C. The unique composition of GBCO codoped with Fe and Cu ions in the Co sites exhibited reduced TEC and enhancement of electrochemical performance and good chemical compatibility with CGO, and this composition is proving to be a potential cathode for IT-SOFCs. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Intermediate-temperature solid oxide fuel cells (IT-SOFCs) with high working efficiency at 500–750 °C have gained considerable importance in the area of power fabrication [1–4]. Recently, simple perovskite-type mixed ionic-electronic conducting (MIEC) oxides, such as doped BaCoO3, LaCoO3 and LaFeO3 have attracted attention as candidate cathode materials. In MIEC oxides, the penetration of conducting oxygen-ions into the electrode bulk greatly increases the active sites for oxygen-reduction, which lowers interfacial polarization [5]. More recent studies on new types of MIEC oxides, i.e. cation ordered LnBaCo2O5+d (Ln = Gd, Pr, Y and La) with two dimensional ion-diffusion channels have shown them to be attractive potential cathode materials for IT-SOFCs. In particular, GdBaCo2O5+d (GBCO) has been reported to exhibit ionic conductivity due to the oxygen vacancies that are mainly located in the rare-earth planes [GdO]x and high electronic conductivity due to the metal– insulator transition (at 87 °C) [6–8]. The GBCO exhibits a high thermal-expansion coefficient (TEC) 20.1  10 6 °C 1 [9], as commonly observed in cobalt-based oxides, which is undesirable for practical use as a cathode for SOFCs [10]. Hence, a systematic investigation on the optimization of TEC and electrochemical performance is essential for the double-

* Corresponding author. Tel.: +82 42 350 4118; fax: +82 42 350 3310. E-mail address: [email protected] (D.K. Kim). 1 Equal contribution of work. 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.08.054

perovskite oxides to be used as potential cathodes in IT-SOFCs. In the present study, oxides of Fe, Ni and Cu doped GdBaCo2O5+d were synthesized via a citrate combustion method. The thermal and chemical compatibilities of the cathodes with Ce0.9Gd0.1O1.95 (CGO) and the area specific resistance (ASR) by electrochemical impedance spectroscopy were studied. A single-cell electrochemical performance of the FC-GBCO cathode on the CGO electrolyte support and Ni-CGO as the anode was also investigated. 2. Experiment Double-perovskite oxides of GdBaCo2/3Fe2/3Ni2/3O5+d (FN-GBCO), GdBaCo2/3Fe2/3Cu2/3O5+d (FC-GBCO), GdBaCoCuO5+d (C-GBCO) and pristine GdBaCo2O5+d (GBCO) were synthesized via the citrate combustion method. Analytical grade Gd(NO3)3
6H2O (>99.9%), Ba(NO3)2 (99 + %), Co(NO3)2
2.5H2O (98 + %), Fe(NO3)3
9H2O, Ni (NO3)2
6H2O and Cu(NO3)3
6H2O were used as precursors for the synthesis of oxide powders, and citric acid (99%) was used as fuel for the combustion reaction. Initially, stoichiometric amounts of the precursors were dissolved in deionized water with constant stirring, and then citric acid was added to the clear solution. The resultant transparent solution was heated at 70 °C until sufficient water had evaporated, and a transparent gel was obtained. The gel was heated at 125 °C in an oven for 24 h. The dried gel was pre-fired at 250 °C and subsequently calcined at 400 °C for 10 h and 900 °C for 20 h to yield the desired crystalline oxide powders.

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The phase of synthesized powders was characterized with an Xray diffractometer (XRD) (Rigaku, D/MAX-IIIC X-ray diffractometer) equipped with Cu-Ka radiation (k = 0.15406 nm). The TEC for each rectangular-shape pellet sintered at 1000–1100 °C for 2 h was measured using a dilatometer (NETZSCH DIL402C) from 30 to 900 °C. The cross-sectional microstructures of the cathode coated on the CGO pellets were characterized by a scanning electron microscope (FE-SEM Philips XL30 FEG). The symmetrical half-cells were fabricated by screen-printing the cathode inks, prepared by mixing the cathode powders with ethyl cellulose, a-terpineol, and dibutylphtalate, on either side of the dense sintered CGO pellets to form a working electrode (WE) and a counter electrode (CE). The as-fabricated half-cells were sintered at 950 °C for 2 h in air to form an effective cathode surface area of 0.385 cm2 and a thickness of 15 lm. Pt paste (ESL) was applied to the edge of the same side of the WE on the electrolyte to act as a reference electrode (RE). The ASR measurement was carried out using impedance spectroscopy (Solartron 1260 impedance/Gain-phase analyzer) in the frequency range of 106–10 2 Hz with a voltage amplitude of 10 mV as a function of temperature (600–800 °C) in flowing air. A single-cell was fabricated by initially screen-printing NiO– CGO (in a weight ratio of 65:35) as the anode on one side of the dense CGO electrolyte pellet (300 lm thick) and was sintered at 1300 °C for 2 h followed by FC-GBCO cathode coating on the opposite side. The single-cell was then sintered at 950 °C for 2 h. The single-cell was sealed at one end of an alumina tube using ceramic adhesive. A Keithley 2400 Sourcemeter was used to measure I–V polarization under flowing humidified H2 (3% H2O) as fuel and air as oxidant.

3. Result and discussions Fig. 1a shows the XRD patterns of the doped GBCO powders calcined at 900 °C for 20 h. The XRD patterns of pristine and doped GBCO oxides exhibit phase pure, high crystalline double-perovskite structure with no peaks attributable to impurities detected [11–13]. All the diffraction peaks of GBCO and FN-GBCO can be indexed based on the JCPDS #53–0135 with an orthorhombic symmetry, space group Pmmm. On the other hand, FC-GBCO and CGBCO exhibited tetragonal symmetry, space group P4/mmm [11,13]. The unit cell parameters for tetragonal symmetry FC-GBCO are a = 3.885 Å, b = 3.884 Å and c = 7.635 Å, and the cell volume is 115.255 Å3. Tarancón et al. [13] described the high-temperature phase transition of GdBaCo2O5+d as occurring at ca. 475–500 °C and corresponding to a symmetry change from orthorhombic Pmmm to the tetragonal P4/mmm phase. Taskin et al. [12] observed that for values of oxygen concentration below d < 0.45, GdBaCo2O5+d shows tetragonal symmetry while above it (0.45 < d < 0.60) becomes orthorhombic (at room temperature). In the present study, the Cu or Fe and Cu doped GBCO retained the tetragonal symmetry at room temperature, which involves oxygen stoichiometry below d < 0.45. Fig. 1b shows the thermal-expansion curves for doped GBCO cathodes measured from 30 to 900 °C. The inset Table in Fig. 1b shows the average TEC values of the doped GBCO cathodes for different temperature ranges, 100–350, 600–900 and 30–900 °C, and compares the calculated TEC values with the reported TEC values of GBCO by Li et al. and Tarancón et al. [9,13]. The observed reduced TEC for doped GBCO cathodes compared with GBCO would be due to the substitution of Co by Fe, Ni and Cu cations that contribute to a reduced effect of both the spin-state transitions of Co3+ and the formation of oxide ion vacancies. Similar trends with the replacement of Co ions by other transition metal ions leads to a decrease in TEC have been observed with various cobalt-based sys-

Fig. 1. (a) XRD patterns of doped GBCO powders calcined at 900 °C for 20 h and (b) Thermal-expansion (dL/L0) curves of doped GBCO in the temperature range of 30– 900 °C in air. Inset Table shows the TEC values for doped GBCO.

tems, including Nd0.6Sr0.4Co0.8M0.2O3 d (M = Ti, Cr, Mn, Fe, Co and Cu) [14] and La1 xSrxCo1 yFeyO3 d [15] in the previous reports. The larger TEC values with the cobalt-based perovskite oxides can be attributed to the electronic spin-state transition associated with low-spin-state (t62g e0g ) to high-spin-state (t42g e2g ) of Co3+ ions, and the high spin-state ion is larger in size compared to the lowspin-state ion [16]. Alternatively, the formation of oxide ion vacancies results in a reduction of the transition metal ions and a consequent increase in the ionic size and a decrease in the overall lattice binding energy [13,17,18]. Thus, in the present study, the substitution of Fe, Ni and Cu cations for Co ions is advantageous to achieve the reduced TECs of the doped GBCO oxides. The calculated ASR values from the interface resistances of the impedance spectra are shown in Fig. 2. An inset in Fig. 2 shows a typical impedance spectrum for the symmetrical cells fabricated with the FC-GBCO oxide on the CGO electrolyte, measured at 650, 700 and 750 °C. The intercepts of the semicircle on the real axis at high-frequency region represent the total ohmic resistance of the electrolyte and current collector wires. The resistance between the two intercepts of the semicircle at low-frequency and the high-frequency regions on the real axis corresponds to the polarization resistance of the cathodes. In Fig. 2, the Fe and Cu

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Fig. 2. Arrhenius plots of log ASR for the symmetrical cells of doped GBCO/CGO at interface measured in air. The inset shows the SEM image of the cross-sectional view of an interface between FC-GBCO cathode and CGO electrolyte substrate. The inset shows the impedance spectrum for the symmetrical cell fabricated with FCGBCO cathode on the CGO electrolyte.

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Fig. 3. Electrochemical performance of the FC-GBCO/CGO/Ni-CGO cell under humidified H2 fuel and air oxidant at different temperatures.

4. Conclusions co-doped GBCO cathode on the Ce0.9Gd0.1O1.95 (CGO) electrolyte showed ASR values of 0.165 X cm2 at 700 °C and 0.048 X cm2 at 750 °C, which are equivalent and lower than the interface resistances of the GBCO cathode at and above 700 °C. The log ASR curve for the GBCO cathode deviates towards higher values. Tarancón et al. reported a similar deviation behavior for the GBCO cathode [13], which was attributed to the decrease in oxygen conductivity above the temperature (700 °C) as the oxygen stoichiometry reaches d < 0.25. On the other hand, in FC-GBCO cathode, the dopant ions Fe and Cu retain the oxygen stoichiometry within the range of 0.25 < d < 0.45, even above 700 °C, and thus a linear behavior would be observed. The FC-GBCO cathode revealed equivalent or better electrochemical properties at and above 700 °C with a reduced TEC value of 14.6  10 6 °C 1 compared with GBCO. A typical SEM image of the cross-sectional view of an interface between the FC-GBCO cathode and the CGO electrolyte substrate, sintered at 950 °C for 2 h, is shown as an inset in Fig. 2. The SEM images expose thin layers of cathodes (9–13 lm thick) with a highly porous morphology that ensures good gas diffusion and the presence of good connectivity at the interface between a porous cathode and a densified electrolyte. The chemical compatibility of cathodes with CGO was investigated by mixing the doped GBCO oxides with CGO in a weight ratio of 1:1 and calcining the mixture at 1000 °C for 4 h. XRD investigations confirm the individual phases of the doped GBCO oxides and CGO even after calcination. However, above 1000 °C, impurity phases attributed to the decomposition of the GBCO phase to the GdCoO3 and BaCoO3 phases have been observed. Fig. 3 shows the I–V polarization performance of the electrolyte supported single-cell configuration of FC-GBCO/CGO/Ni-CGO in the temperature range from 550 to 700 °C. The obtained maximum power density was 435 mW cm 2 at 700 °C for the FC-GBCO/ CGO/Ni-CGO single-cell, which is comparably higher than the reported values of 150 °mW °cm 2 for the Ni-YSZ/YSZ/GBCO anode supported single-cell [19], 70 °mW °cm 2 for GBCO/GBCO-LSGM/ LSGM/LDC/Ni-CGO [20] and 300 mW cm 2 for the Ni–SDC/SDC/ SBCO [21] electrolyte supported single-cells at 700 °C. These results demonstrate that the FC-GBCO cathode exhibits reduced thermal expansion and high electrochemical performance in intermediate-temperature regime of 700 °C with the CGO electrolyte.

The double-perovskite FN-GBCO, FC-GBCO C-GBCO and GBCO cathode materials were synthesized via the citrate combustion method. The FC-GBCO cathode exhibits a relatively reduced TEC value of 14.6  10 6 °C 1 compared with that of GBCO. The doping ions Fe and Cu retain the oxygen stoichiometry within the range of 0.25 < d < 0.45, even above 700 °C and possess equivalent or better ASR values compared with GBCO. The maximum power densities of the electrolyte supported single-cell configuration of FC-GBCO/ CGO/Ni-CGO attained 233 and 435 mW cm 2 at 650 and 700 °C, respectively. The achieved power density and reduced TEC are encouraging and indicate that a 0.666 equiv-molar composition of Fe and Cu co-doped GBCO oxide is a proving cathode material for IT-SOFCs. Acknowledgements This work was financially supported by the Korea Research Foundation Grant funded by the Korean government (MOEHRD; KRF-2008-005-J00903). This work was partially supported by Brain Korea 21 (BK21) program from Korean Ministry of Education. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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